In the discussion that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art against the present invention.
In particular in the automotive industry, it has turned out that the fuel efficiency of a motor may be substantially improved by using light weight cylinder blocks and coating the inside of the motor cylinders with a hard and wear-resistant coating having favorable gliding properties because the light weight material of the cylinder block as such would have a poor wear resistance.
However, there is the general problem of a sufficient adherence of such coating to the cylinder surface without any flaking during a desired life time of such motor. In order to improve adherence of coatings on motor cylinders, the cylinder surface should not be too smooth and there have been developed many different methods and devices to increase the roughness of the cylinder surface in order to improve the adherence of such coatings. In particular the application of high pressure water jets is one of the methods which have shown to create a surface roughness and surface structure which is suited to provide a good adherence for cylinder coatings. However, water jetting is rather cumbersome and expensive and therefore less suited for industrial mass production.
As an alternative to water jetting which generates a rough but irregular surface, there have been developed methods to provide well defined microstructures by micro milling, i.e. by rotating cutting elements having a well defined geometry of rather small dimensions of typically less than one mm. Such cutting elements may generate surface structures and in particular grooves in the μm-regime (order of magnitude), i.e. with dimensions between 10 and 1000 μm, or more specifically between 100 and 500 μm.
Micro milling is typically performed with a rotating tool having cutting teeth generating one or several spirally wound thread-like grooves. In order to further improve adherence of coatings. It is also known to provide such grooves with an undercut cross section.
The generation of undercut grooves may also include plastic deformation of sides and edges of the grooves or of ridges remaining between adjacent grooves. Also the direct milling of undercut grooves by cutting teeth having a corresponding shape and arrangement is already known in the art.
Examples of corresponding tools and methods are for instance disclosed in US 2005/0064146, EP 09 776 027 and DE 10 2009 028 040.
The known tools and methods for the direct milling of undercut micro grooves are designed to generate a thread like structure by means of cutting teeth following a spirally wound path. Thereby, the micro groove structure is generated by means of a plurality of different cutting teeth successively engaging the same groove upon rotation of the tool, thereby generating the final shape and cross section of the grooves.
This necessarily implies that for generating a completely machined cylinder surface the tool is rotated while being be axially fed with a rather low axial feed rate along the total length of the cylinder, because the pitch of the thread like grooves should not exceed a maximum value in the order of two to five times the width of the individual grooves. Otherwise the adherence of the coating may be reduced due to wide smooth surface portions left between adjacent grooves. This operation requires a large number of revolutions of the tool before the structuring of the cylinder surface is completed.
To improve the cycle time of such tools, in particular where the tool has to be withdrawn through said cylinder after completing the groove structure, it is already known to arrange the respective cutting elements offset to one side of the tool body and reducing the radial extension of the tool on the opposite side, i.e. reducing the tool diameter. It is then possible to exit a hollow cylindrical space without engaging the cylindrical wall, after the surface milling is finished. That is, after finishing the milling process by rotating and axially feeding the milling tool into and through the cylinder, the tool body is radially offset in order to bring the cutting elements out of alignment with the cylinder wall, and then axially withdrawn through the cylinder. Of course, this also implies that the rotation of the tool body during the milling process is effected about the cylinder axis which in turn is offset from any symmetry axis of the tool body in order to allow the tool body and cutting elements to exit the hollow cylindrical space without engaging the cylinder wall.
The microstructures have typical dimensions well below 1 mm, i.e. the depth and width of the grooves provided in the cylindrical surface is below 1 mm, while the pitch may be about twice or three times the groove width and could be up to the order of or slightly larger than 1 mm.
This does of course not exclude the provision of methods and corresponding tools and cutting elements generating corresponding structures with larger or smaller dimensions.
A prior art tool comprising the aforementioned features is known from DE 10 2009 028 040 A1. The cutting elements within this tool are fixedly mounted, i.e. while being still adjustable, they remain in a fixed position when the tool enters a hollow cylindrical body, whereupon the tool is rotated about an axis which is offset from the center of the tool while being axially fed. Finally the whole tool is radially shifted to bring the cutting elements out of alignment with the wall of the work piece and then axially retracted.
As already mentioned, the respective cutting elements are provided to generate spirally wound grooves similar as a thread with a very small pitch. This requires many revolutions of the tool within the hollow cylindrical space until the structured surface is completed.